1. Technical Field
This invention relates generally to fragment penetration and fire shields, and, more particularly, to penetration and bum-through resistant fabric structures and materials used to absorb energy, arrest projectiles, and act as a fire shield.
2. Description of the Related Art
Over the years several civil aircraft accidents having catastrophic consequences have resulted from damage to critical aircraft components by flying engine fragments produced by an in-flight engine failure. Four systems are critical for continued safe operation and landing of an aircraft: the flight control lines, the fuel lines, the engines, and the pressure boundary. The flight control lines and fuel lines, which are separated spatially in the aircraft and redundant, must not be severed by engine fragments. Likewise, second or third engines need to be operational and thus must not be incapacitated by fragments from a failed engine. Finally, compromise of the pressure boundary (holes and tears in the fuselage wall, for example) at typical cruising altitudes could be catastrophic. The desire to provide ballistic protection at minimum weight and cost and to reduce still further the risk of a catastrophic accident from in-flight engine failure requires low weight with high ballistic properties.
Likewise, several civil aircraft incidents having catastrophic consequences have resulted from combustible material used in insulation blankets and the inability of these materials to act as a fireblock and permit time for passengers to evacuate a burning aircraft. Compounding the problem is the fact that most of the objects inside the aircraft, i.e., seats, walls, bins, are polymeric and may burn and/or decompose into flammable gases when exposed to heat.
Materials supporting combustion were blamed for the Swiss Air incident near Nova Scotia and resulted in a directive by the FAA to replace insulation in all aircraft that contained metallized mylar. Another requirement is for a fire blocker that prevents fire outside the aircraft from penetrating into the cabin for a minimum of 5 minutes to allow evacuation of the occupants.
To mitigate damage caused by projectile penetration, many types of barrier systems have been constructed. Steels have traditionally been the material of choice for land vehicle armor. As shown in FIG. 1a, hard steel surfaces produce large stresses in perpendicularly impacting projectiles, blunting the leading edges and/or breaking them into two or more pieces. Further, as illustrated in FIG. 1b, steel surfaces are effective in deflecting projectiles striking the surfaces at an angle. Besides their effectiveness in defeating impacting projectiles, steels are inexpensive relative to other materials and are excellent structurally; being weldable, durable, formable, corrosion resistant, compatible with other structural components, and field repairable. The main drawback of steels is their high density, which results in heavy armor structures and renders them especially unsuitable for use in aircraft.
Ceramics have also been used in the construction of barrier systems. Ceramics make good armor and in many instances outperform conventional rolled homogeneous steel armor. High compressive strength allows ceramics to exert large stresses on high speed impacting projectiles, stresses that act to deform, deflect, and fracture the projectile as shown in FIGS. 1a and 1b, as well as eroding the leading edge of penetrating projectiles and eventually reducing them to particles, as illustrated in FIG. 1c. This is very effective against rapidly moving bullets. However, the ability of ceramics to deform, deflect, fracture, and erode a projectile decreases as the velocity of the projectile decreases, because at low projectile velocities fracture of the ceramics occurs at very early times during traverse of the projectile, thereby increasing the probability that the projectile will succeed in piercing the barrier system.
Polymeric fibers are competitive with metals and superior to ceramics at lower projectile velocities. These fibers deform to absorb the kinetic energy of projectiles striking them, slowing or stopping the projectile. However, polymeric barriers used with aircraft have been primarily positioned within the engine nacelle. However, when a rotor disk bursts, the containment structure may be defeated, allowing debris to be projected outwards in the direction of the fuselage and wings.
Likewise, modern ground transportation continues to evolve towards vehicles of lower and lower weight. Demands for speed and maneuverability as well as fuel efficiency require a light vehicle weight. Conversely, impregnability to projectile penetration traditionally required heavy armor, especially for military vehicles. Thus, a trade off must be reached between maintaining a low enough weight to be practical while ensuring protection of critical components and the safety of the passengers.